The summer 2009 field season was the third of six sampling efforts to
assess the distribution and abundance of redband trout in the six
interior basins of Oregon’s high desert. Abundance was evaluated at the
Species Management Unit (SMU) level for
the Catlow Valley, Goose Lake, Malheur Lakes, and Warner Valley SMUs, and at the
population level in Chewaucan and
Fort Rock SMUs.

Sites were randomly selected using
EPA's Generalized Random Tessellation Stratified design
protocol. We targeted 30 sites for the Goose and Warner Lakes SMU’s, 40 sites in the Malheur Lakes, and 20 sites
in the Catlow Valley SMU. We also targeted 30 sites for each population assessed in the Chewaucan and Fort Rock basins. Actual number of sites sampled approached but
did not completely meet these targets. However, we were able to sample nearly
75% of selected sitesacross
the range of each SMU. Extensive efforts (public meetings, letters, personal
phone calls and visits) were made to obtain permission to access sites
located on private property. Field methods used to sample each site were
the same as those used in
2007.
Length frequency analysis was used
to categorize redband trout as
either age-0 or age-1+ fish. Because they are not completely vulnerable to our gear, age-0 fish were not
included in our estimates. Separate age-class designations were made in each of
the six SMUs. Population estimates for age-1+
redband trout in each SMU were extrapolated from depletion estimates of
fish density at sample sites. A calibration factor was applied to
estimates to adjust for the bias associated
with depletion estimates. The calibration factor was calculated as the mean ration between depletion and mark-recapture
estimates at a subsample of sites where both methods were employed.

Depletion estimates were completed at 218 sites across the 2,340 km sample frame (potential summer distribution)
of Great Basin redband trout. The final calibrated population estimates ranged from
16,000-400,000 at
the SMU scale and from 1,000-150,000 at the
population-scale. The precision
of these estimates ranged from ±15% to ±45% and varied relative to the between-site
variability (CV) and sample size for each sample stratum.
Densities estimated at individual sites
were highly variable (density histograms).